It is normally difficult to render a halftone image using a second halftone process. Often, the two halftones interfere, causing a distortion in the form of a low frequency binary banding pattern, called moiré, which appears as alternating light and dark bands, or patches, in the second generation halftone image. The normal procedure to reduce or eliminate this moiré is by applying a low pass filter to the original halftone image, thus smoothing or eliminating the binary pattern. Such low pass filters may be optical or digital, and smooth or eliminate the original halftone pattern. The filtering process causes blurred edges and loss of fine details. This additional digital image process considerably slows the rendering process. Some improvements to the filtering process attempt to preserve sharp edges, however, these improvements only help to preserve hard, distinct edges and do nothing to preserve fine details.
U.S. Pat. No. 4,907,096 for Image processing, to Stansfield et al., granted Mar. 6, 1990, describes a method for processing a screened separation, which includes use of an analyzing scanner for scanning the separation at a resolution higher than the screen ruling to generate a binary value for each high resolution pixel of the separation, and uses a descreening system to which the signals from the processor are fed. The descreening system generated signals representing a descreened version of the original separation. The descreening system divides the binary representation into a number of blocks of high resolution pixels, each block constituting a low resolution pixel. The binary values in each block are summed to generate the resultant single value for the block.
U.S. Pat. No. 5,027,078 to Fan, for Unscreening of stored digital halftone images by logic filtering, granted Jun. 25, 1991, describes a method of unscreening a digitally created halftone image in order to reconstruct a continuous tone image, including the determination of the parameters of the halftone screen used to produce the halftone image, logically filtering the halftone image to determine approximate continuous tone levels, and optionally, smoothing the continuous tone levels of the reconstructed image to minimize the quantization errors introduced during the original screening or dithering process.
U.S. Pat. No. 5,239,390 for Image processing method to remove halftone screens, to Tai, granted Aug. 24, 1993, describes a method for reproducing an image which includes scanning an original image to produce a digitized image, performing a local structure analysis of the digitized image, and selectively applying a descreening filter to a region of the digitized image based on the results of the local structure analysis to remove a specific frequency of the digitized image caused by halftone screens.
U.S. Pat. No. 5,243,444 for Image processing system and method with improved reconstruction of continuous tone images from halftone images including those without a screen structure, to Fan, granted Sep. 7, 1993, describes an image processing system which converts unscreened and other halftone images to continuous tone images. Value data is sequentially generated for successive pixels of a screened or unscreened halftone image. Each image pixel is Sigma filtered with a predetermined set of filter parameters, including the filter window size and a Sigma difference range which is applied to determine which pixels in the filter window are counted in determining average window pixel values. An output continuous tone image containing the Sigma filtered pixels is generated for storage and/or processing to a halftone copy or print.
U.S. Pat. No. 5,339,170, for Image processing system and method employing hybrid filtering to provide improved reconstruction of continuous tone images from halftone screen-structured images, to Fan, granted Aug. 16, 1994, describes an image processing system which converts screen-structured halftone images to continuous tone images. Value data is sequentially generated for successive pixels of a halftone image. An averaging filter is provided for sequentially filtering each pixel in the halftone image in the horizontal image direction in accordance with a first predetermined filter to generate an intermediately filtered image. A pattern matching filter then sequentially filters each pixel in the intermediately filtered image in the vertical direction to generate a hybrid filtered image. The hybrid filter arrangement is then iteratively operated for three additional sets of orthogonal directions, i.e., the vertical and horizontal directions, a first diagonal direction and a second diagonal direction, and a combination of the second and first diagonal directions. The best hybrid image is generated as an output continuous tone image for storage and/or processing to a halftone copy or print.
U.S. Pat. No. 5,343,309 for Image processing system and method employing adaptive filtering to provide improved reconstruction of continuous tone images from halftone images including those without a screen structure, to Roetling, granted Aug. 30, 1994, describes an image processing system which converts halftone images to continuous tone images. It employs an adaptive filter, which processes successive pixels in an input halftone image. The adaptive filter employs a filter which is selected under feedback control from a plurality of filter sets, each having a plurality of filters. The halftone image is also low-pass filtered to generate a first approximation image (FAI). A spatial gradient value is computed for each pixel in the FAI. A control operates the adaptive filter to apply one of the predetermined filters to the current pixel as a function of the associated pixel spatial gradient. An output image from the adaptive filter in a first iteration of the filtering procedure may then be applied to the input of the adaptive filter for a second adaptive filtering iteration. Pixel gradients for the second iteration are computed from the image output from the first iteration. A predetermined number of iterations are performed and the image output from the last iteration is a continuous tone image for system output.
U.S. Pat. No. 5,384,648 for Apparatus and method for descreening, to Seidner et al., granted Jan. 24, 1995, describes a technique for descreening and for performing resolution changes and correction of misregistration on a color halftone image in order to produce a continuous tone color image. The apparatus includes a filter apparatus for removing screen information from the halftone image, including a plurality of different filters, each providing interpolation and screen removal functions, and a controller for selecting per-pixel of the continuous tone image and in accordance with sensed misregistration, and as a function of desired resolution changes, one of the filters for operation on a neighborhood of the pixel.
U.S. Pat. No. 6,101,285 for Filter for producing continuous tone images from halftone digital images data, to Fan, granted Aug. 8, 2000, describes a technique for processing halftone digital image data to generate a corresponding continuous tone image is disclosed, which includes a method and apparatus for filtering a halftone image in two directions, which enables images to be smoothed in the flat regions, while reducing smoothing along edges of the image and eliminating smoothing altogether in the ridges and valleys. This provides for an improvement in image quality without losing the fine details that are often lost using standard filtering techniques.
U.S. Pat. No. 6,163,629 for Method for low complexity memory inverse dithering, to Cheung et al., granted Dec. 19, 2000, describes a method and apparatus for inverse dithering a dithered image. The system includes a plurality of digital filters, which are organized in a preselected order. A selection module enables the selection of one filter from the plurality of filters according to the preselected order to filter a presently selected portion of the dithered image. The selected portion of the dithered image is then processed based upon the selected filter to generate a portion of the inverse dithered image.